In order to get high performance in a conventional radio frequency (RF) receiver, it is often required that the input signals to internal components in the RF receive path be centered on zero volts (0 V). Even a small direct current (DC) offset voltage at the input of a component may have a large effect on the dynamic range of the component.
For example, many RF receivers comprise an analog-to-digital converter (ADC) that converts an analog pass band signal by using a high sampling frequency. One common ADC component comprises a sigma-delta (ΣΔ) modulator block followed by a decimation filter block. After the A/D conversion, the frequency of the pass band signal is decreased to the level of the base band, but the sampling frequency still remains high. Excessive sampling is reduced by the decimation filter, which also improves the signal-to-noise ratio.
A cascaded integrator comb (CIC) decimation filter is commonly used in many receive paths. In decimation, the sampling period is increased and the sampling frequency is decreased. This reduces the number of data points that must be processed and stored in each unit of time. A CIC decimation filter reduces sampling of a signal by a high coefficient. The CIC decimation filter typically comprises a chain of integrators, a decimation circuit, and chain of successive comb filters.
However, a problem associated with the decimation filter employing a chain of integrators is that the long-term expected value of input data must be zero. This means that data entering the CIC filter must not contain a DC offset voltage. Even a minor DC offset in the input data may cause saturation of the integrators and a functional error in the decimation filter.
Unfortunately, many of the analog circuits in the receive path preceding the decimation filter generate DC-offset voltages, either individually or by combined effect. This DC-offset may reduce the dynamic range of the decimation filter, an analog-to-digital converter, or another component in the receive path. This makes it very desirable to minimize DC-offset voltages.
Therefore, there is a need in the art for an improved radio frequency (RF) receiver. In particular, there is a need in the art of an RF receiver that minimizes DC offset voltages in the receive path, particularly at the inputs to an ADC block and a decimation filter.